A wheel tractor-scraper is a machine employed in various industries, such as agriculture, construction and mining to load, haul, eject and spread layers of earth. Such machines are particularly suited for applications such as roadway construction and site preparation, where material needs to be removed or added while creating or maintaining grade and hauling occurs over a distance. Conventional wheel tractor-scrapers typically include a tractor portion having a forward frame member that supports an operator station and a propulsion power source operatively coupled to the drive wheels of the machine. An articulated joint couples the tractor portion to the rear scraper portion of the machine. The scraper portion has a rear frame member that supports both a bowl for collecting and hauling material, and the rear wheels. During operation, the bowl is typically lowered to engage the ground along a cutting edge that is driven forward by the machine, thus, scraping the earth and loading the bowl. These machines may have an earth-moving work tool, such as an elevator, conveyor, auger, or spade, associated with the bowl to facilitate penetration, loading, and/or unloading of the material to be transported.
One problem with wheel tractor-scrapers is that the articulated joint that couples the tractor portion to the rear scraper portion of the machine may transfer a great deal of pitch and bounce causing shock and vibration to propagate through the machine to the operator when the machine contacts bumps and/or holes along a driving path.
The disclosure of U.S. Pat. No. 3,311,389 provides a system for control of pitch and bounce in tractor-trailer machines. Thus, in the '389 patent a tractor-trailer hitch is shown in which vertical motion is permitted, but limited and cushioned by a hydraulic cylinder connected between the units and associated with gas over oil accumulators and a fixed restrictive element (e.g., a fixed fluid orifice) to provide the desired spring rate. Given that this system is to reduce shock and vibration, it is desired to keep the hydraulic cylinder's rod situated near a centered extension point during travel time so that it may extend or retract as the wheels of the scraper portion engage bumps or holes in the driving path. To slow movement of the cylinder rod so that it is less likely to “top out” or “bottom out” when bumps or holes in the driving path are engaged, a fixed restriction is provided in the fluid line, thus slowing flow of fluid to or from the cylinder. Shortcomings of the '389 patent were improved upon by U.S. Pat. No. 3,430,657, which provides a balancing of gravitational forces in a vertically disposed valve spool.
However, these references both only provide a fixed restrictive element, which may have only one ideal loading weight to minimize vibration and shock propagated through the machine. Thus, it is desirable to provide a system that improves upon these and other shortcomings of an articulated hitch system, as discussed above, and allows for tuned vibration damping at multiple loading levels of the machine.